An air blower equipped with a brushless DC motor is used in, for example, a ventilator for exhaust of ceiling built-in type and the like, a ventilator for intake of ceiling built-in type and the like, an air blower, a humidifier, a dehumidifier, a refrigeration device, an air conditioner, and a water heater. In addition, in recent years, an air blower mounted on an electric device such as a ventilation device has been desired not to be affected by pressure loss due to ductwork form, outside wind pressure, and change of pressure loss due to the clogging of filter or the like. An air blower with good controllability, capable of ventilation with a constant air volume of optimum air volume depending on room conditions has been sought.
Conventionally, this kind of air blower has been known for the configuration disclosed in PTL 1. In the following, the conventional air blower will be described with reference to FIG. 6, which is a block diagram illustrating a configuration of a conventional air blower.
As illustrated in FIG. 6, centrifugal type air blower 110a is equipped with brushless DC motor 111, and inverter circuit 104 includes an upper stage and a lower stage. In addition, centrifugal type blower 110a is driven by inverter circuit 104 including a bridge connection of upper stage side switching elements 107u, 107v, and 107w, and lower stage side switching elements 108u, 108v, and 108w. Applied voltage detector 115 detects the voltage value of DC power source 109 to be applied to inverter circuit 104.
Drive logic controller 105 performs PWM control of DC power supply 109 by upper stage side switching elements 107u, 107v, and 107w or lower stage side switching element 108u, 108v, and 108w. In addition, drive logic controller 105 energizes driving coils 102u, 102v, and 102w of brushless DC motor 111 in full wave sequentially in a specified direction and order.
Duty indicator 106 indicates the ON/OFF duty when performing PWM control of upper stage side switching elements 107u, 107v, and 107w or lower stage side switching elements 108u, 108v, and 108w. Equivalent voltage detector 114 detects the equivalent voltage applied to driving coils 102u, 102v, and 102w by multiplying the voltage value that is detected by applied voltage detector 115 by the ON/OFF duty indicated at duty indicator 106.
At the time of a predetermined voltage of an equivalent voltage detected by equivalent voltage detector 114, an output of duty indicator 106 is changed. Thereby, the ON/OFF duty of the PWM control becomes variable, and the average current supplied to inverter circuit 104 is controlled to a predetermined current value.
As a result, brushless DC motor 111 has the rotating speed-torque characteristic that the shaft torque is further increased along with the elevation of the rotating speed without being affected by variations in power supply voltage. Therefore, an air blower and an electrical device are available which have wind volume-static pressure characteristic that wind volume does not change significantly even when static pressure such as pressure loss changes.
In such a conventional brushless DC motor controller, the average current value supplied to inverter circuit 104 is determined from the equivalent voltage detected by equivalent voltage detector 114. The ON/OFF duty when performing PWM control is made variable so that the average current value is achieved. For this reason, there has been a problem that the motor current proportional to the generated torque of brushless DC motor 111 cannot be controlled, and that a deviation occurs also in the torque.
That is, even if an equivalent voltage being in a state where the voltage of DC power source 109 applied to inverter circuit 104 is large, and ON/OFF duty is small, and an applied voltage being in a state where the voltage of DC power source 109 is small, and ON/OFF duty is large, are equal to each other, the average current value supplied to inverter circuit 104 is different in each of the states. Therefore, there is a problem that a generated torque in brushless DC motor 111 also becomes different in each of the states.